1. Field of the Invention
The present invention generally relates to temperature sensors. More specifically, the present invention relates to the temperature characterization of integrated circuits, that is, the checking of the temperatures at which the circuit operates properly.
2. Discussion of the Related Art
During the final testing steps of an integrated circuit, to check or determine the proper operating temperature range, the integrated circuit is submitted to a specific testing.
FIG. 1 schematically illustrates a device used to test the operating temperature range of a completed integrated circuit (that is, in a package). An integrated circuit 1 to be tested is placed on a support 2 and put in electric connection with a testing tool 3 by a conductive connection 4. A testing temperature is obtained by locally creating a heated or cooled down atmosphere at the desired temperature, projected by a pulsed air pipe 5 as close as possible to circuit 1. Once an atmosphere has been created at the desired temperature, the operation of the integrated circuit is tested by means of external tool 3.
The test is repeated for different temperatures to check, for example, the proper operation of the circuit in the provided range.
A problem posed is to know the real temperature of the integrated circuit and, more specifically, temperatures of semiconductor junctions. On the one hand, the integrated circuit is generally placed in a package which partially protects it from external temperature variations. On the other hand, in operation, the different circuit portions heat differently, which may alter the measurements.
Generally, a sensor integrated with the circuit is used to determine a voltage of which the variation according to temperature is known.
FIG. 2 illustrates such a sensor conventionally used to determine the real temperature of the integrated circuit in an operation test. The sensor includes a PNP-type bipolar transistor 6, integrated in a semiconductor wafer as integrated circuit 1 to be tested (not detailed). Bipolar transistor 6 is diode-connected, its base and its collector being interconnected to a voltage reference rail GND. The emitter of transistor 6 forms an input/output terminal 7 of the sensor. Terminal 7 is connected by connection 4 to external tool 3 which includes a current source 8 to be interposed between emitter terminal 7 and a high voltage supply rail VDD with respect to rail GND. The voltage on terminal 7 is measured, still with respect to the same reference GND.
The voltage thus sampled is the base-emitter voltage VBE of transistor 6. The variation of this voltage according to temperature T of the semiconductor substrate—typically silicon—in which the base-emitter junction is integrated is known according to the following formula:I=Isat.exp(qVBE/nkT), where
I is the current imposed by source 8 on emitter 7;
Isat is the saturation current of the base-emitter junction of transistor 6;
q is the atomic charge;
k is Boltzmann's constant; and
n is the ideality factor of transistor 6.
The variation of current I being imposed by external tool 3 and voltage VBE being measured, internal temperature T can be determined if the saturation current of junction Isat and the ideality factor are known.
A disadvantage of conventional test cells such as that illustrated in FIG. 2 is that it is necessary to know the characteristics of transistor 6 forming the sensor and especially its saturation current Isat. In fact, a range of ideality factors for which current I provided by source 8 provides a result which is assumed to be valid has to be set.
Several distinct tools must then be available according to the ideality factors. Further, there now does not exist any external tool enabling processing all possible ideality factor values. For example, a currently-used testing tool is provided for an integrated circuit having an ideality factor which must range between 1.0057 and 1.0125.